As Social and Emotional Learning (SEL) expands to focus on adolescent populations, the broadly accepted theoretical framework put forth by the Collaborative for Academic, Social, and Emotional Learning (CASEL) should be empirically tested for measurement utility. Using longitudinal data from the 4-H Study of Positive Youth Development, we first tested and validated the five (self-awareness, self-management, social awareness, relationship skills, responsible decision-making) SEL factor model using confirmatory factor analysis (CFA) in a normative sample of 1,717 U.S. fifth grade youth. The model was then subjected to longitudinal measurement invariance testing using CFA models that included the sixth- and seventh-grade samples to confirm SEL as a robust model across these grades. Validity was further evidenced through relation of the SEL model to important youth outcomes (e.g., academic achievement). Relations were significant and in the expected direction. Implications for application of the model to adolescent development are discussed.